In order to enhance the durability of a magnetic recording medium, there has been employed a method, for example, of providing an improved binding of magnetic powder to a binder using a binder having a high compatibility with the magnetic powder, thus improving dispersibility and packing properties to decrease the rubbing-off of the powder. In this method, it has been proposed that there be employed as a binder a polymer which is modified by the polymerization of its copolymeric ingredients highly compatible with the magnetic powder, thereby improving the binding property of the binder to the magnetic powder. This method, however, requires that all the copolymeric ingredients are capable of being polymerized. Even if this requirement is met, a binder containing a desired amount of copolymeric ingredients cannot be produced or a block copolymer is produced in the absence of a proper relative polymerzation ratio, so that conditions of manufacture are significantly limited. This method is accordingly less valuable and less practical in industrial applications.